1. Field of the Invention
The present invention relates to a correcting means for correcting "variations" in reference resistance and constant of a thermistor as a temperature sensor in a thermometer to detect a temperature by an oscillation frequency of a temperature sensitive oscillator which is determined by a resistor of the thermistor.
2. Description of the Prior Art
Along with rapid advances in precision and stability of thermistors, thermistors have received a great deal of attention as temperature sensors in high-precision thermometers or clinical thermometers in recent years. When a thermistor is used as a temperature sensor in a thermometer and a clinical thermometer (these types of thermometers will be referred to as thermometers hereinafter), temperature sensitivity characteristics are determined by a reference resistance inherent to each thermistor and a thermistor constant (to be referred to as a B constant hereinafter). It is impossible to set the standard reference resistance and the standard B constant of an individual thermistor during the manufacture thereof from the viewpoint of industrial basis. Even if the thermistors of the identical standards are used, "variations" occur in the temperature sensitivity characteristics of the individual thermistors. As is well known, since the temperature sensitivity characteristic curve changes exponentially, it must be corrected linearly.
Thermistors are compact and inexpensive and can be easily handled as compared with other temperature measurement sensors. When the "variation" and "linear correction" problems are solved, they are suitable as temperature sensors of high precision and good stability since they can be manufactured as temperature sensors of a wide measurement range at low cost in mass production lines.
An arrangement and characteristics of a CR oscillator widespread and also used as a temperature sensitive oscillator of the present invention will be described with reference to FIG. 1.
FIG. 1 is a circuit diagram showing a typical example of the CR oscillator. Reference numeral 1 denotes a thermistor. A time constant circuit consisting of the thermistor 1 and a capacitor 6, and two CMOS inverters 7 and 8 constitute the CR oscillator. The CR oscillator converts to a change in oscillation frequency a change in resistance which is caused by a change in temperature of the thermistor 1.
In the CR oscillator described above, an oscillation frequency f is given as follows: EQU f=1/2.2C0R (1)
where C0 is the capacitance of the capacitor 6 and R is the resistance of the thermistor 1.
The resistance R of the thermistor at a temperature T(K) is derived below: EQU R=R0 exp{B(1/T-1/T0)} (2)
where R0 is the resistance of the thermistor at a reference temperature T0(K) and B is the thermistor constant (the thermistor constant will be referred to as the B constant hereinafter) representing the sensitivity given by the temperature-resistance characteristics.
Substitution of equation (2) into equation (1) yields the following equation: EQU f=1/[2.2C0R0 exp{B(1/T-1/T0)}]=exp(B/T0)/{2.2C0R0 exp(B/T) }(3)
The number N of pulses generated from the CR oscillator for a unit time t is: EQU N=ft (4)
Equations (3) and (4) yield the following equation: EQU T=B/{(B/T0)-ln (2.2C0R0N)/t} (5)
Equation (5) is used to calculate a temperature T upon detection of the number N of pulses from the CR oscillator during the time t.
The temperature sensitivity characteristic curve of the CR oscillator, changes in temperature sensitivity characteristic curve upon use of thermistors with different characteristics, and "linear correction" and "variation correction" of the temperature sensitivity characteristic curve will be described hereinafter.
FIG. 2 is a graph showing temperature sensitivity characteristic curves of thermistors. The pulse rate (N/t) of the oscillation signal f0 from the CR oscillator is plotted along the abscissa, and the temperature is plotted along the ordinate.
A curve a in FIG. 2 shows the standard temperature sensitivity characteristics of a thermistor thermometer using a thermistor having the rated B constant and a capacitor 6 having the rated capacitance C0. The standard temperature sensitivity characteristics are defined by equation (5). Curves b and c are obtained when thermistors have the standard resistance but B constants are deviated from the rated values, respectively.
As described above, the curve a of FIG. 2 simply represents the relationship given by equation (5). As is apparent from the curve a, equation (5) is a monotone increasing function. An ambient temperature of the thermistor 1 in the CR oscillator is solely determined by the number N of pulse signals for the predetermined time period t (measuring time).
The reason why linear correction is required in temperature measurement using the curve a will be described hereinafter.
In order to measure a temperature (to be described in detail later), the number N of pulses generated by the CR oscillator is counted by a counter. The counter can perform only linear incrementation, so that the count cannot be employed as the temperature data without modification. Counting must be performed on the basis of equation (5) to calculate the temperature T. As a result, an expensive operation circuit is required.
In order to realize an inexpensive thermistor thermometer, the number N of pulses from the CR oscillator must be counted by a counter having a counting characteristic approximating the curve a of FIG. 2, thereby obtaining the temperature T. For this purpose, a simple linearizing circuit is added to the counter to achieve linear correction, and the count of the counter must be used as direct temperature data. U.S. Pat. No. 4,464,067 issued to the present applicant describes a typical linearizing circuit of this type. The linearly corrected results are given by polygonal lines a1, b1 and c1 of FIG. 2 which respectively correspond to the curves a, b and c. The polygonal curves a1, b1 and c1 are obtained without performing variation correction (to be described later), so that their positions and slopes do not change since only linear correction is performed.
Variation correction of the thermistors will be described with reference to FIG. 3. As is apparent from equation (2), the temperature sensitivity characteristics of the thermistor depend on two characteristic constants inherent to the thermistor, i.e., the reference resistance R0 on the B constant B, as described above. The variations in the temperature sensitivity characteristics of the individual thermometers can be accurate when the reference resistance R0 and the B constant B of the individual thermistors used in thermistor thermometers are corrected during the fabrication of the thermistor thermometers.
The correction of the reference resistance R0 will be described first.
The temperature T in equation (5) does not change when a product of the reference resistance R0 and the capacitance C0 of the capacitor is predetermined. This indicates that an error (variation) from the standard resistance R0 can be corrected by adjusting the capacitance C0 of the capacitor. For this reason, in an embodiment to be described later, the capacitor 6 in the CR oscillator of FIG. 1 comprises a variable capacitor, and the capacitance C0 of the capacitor 6 is adjusted so as to correct "variations" in the reference resistance R0 of the thermistor 1. Alternatively, the resistance R0 and the capacitance C0 are fixed, and the time t in equation (5) is adjusted. Therefore, the industrial "variations" in the capacitance C0 in the capacitor 6 in the CR oscillator and "variations" in the reference resistance R0 of the thermistor 1 can be simultaneously adjusted.
Polygonal lines b2 and c2 as the result of correction of the reference resistances are shown in FIG. 3, so that the polygonal lines b1 and b2 of FIG. 2 are shifted by .DELTA.R1 and .DELTA.R2, respectively, thereby correcting them to have the pulse number N0 equal to the polygonal line a1 of the reference temperature sensitivity characteristics at the reference temperature T0.
The principle of correction of "variations" in B constant will be described hereinafter.
When the "variation" error of the B constant of the thermistor 1 is .DELTA.(-1&lt;&lt;.DELTA.&lt;&lt;1) and is used as the oscillation frequency variable element, the relationship between the number N of pulses and the temperature is expressed by equation (5): EQU T=B(1+.DELTA.)/{B(1+.DELTA.)/T0-ln (2.2C0R0/t)N} (6)
Assume that the "variations" in the reference resistance R0 have corrected in accordance with the method as described above and that linear correction has already been performed. Equation (6) varies in accordance with different individual thermistors and presents polygonal lines such as lines b2 and c2, the slopes of which are different due to the different B constants of the individual thermistors from the polygonal line a1 of the standard temperature sensitivity characteristics. The polygonal line b2 is obtained when the error .DELTA. of the B constant is positive, while the polygonal line c2 is obtained when the error .DELTA. is negative.
As described with reference to the curve a of FIG. 2 and the polygonal line a1 of FIG. 3, when the counter is provided with the characteristics of the graph of polygonal line, the curve a can be approximated by the polygonal line to a straight line. However, even if output pulses from the CR oscillator having the temperature sensitivity characteristics given by the line b2 or c2 of FIG. 3 are counted, counts at the respective temperatures excluding the reference temperature T0 differ from each other due to different slopes caused by different B constants of the thermistors. For this reason, the different thermistors show different temperatures although an identical temperature is measured. As a result, variations in B constants, i.e., variations in sensitivity of the thermistors occur.
There are a few conventional techniques for correcting the variations in sensitivity. According to a first technique, a series- and parallel-connected resistor array is connected to the thermistor (to be described later), and the sensitivity is corrected in an analog manner. According to a second technique, sensitivity is corrected by arithmetic operations in accordance with the correction data. A third technique is proposed by the present applicant in U.S. Pat. No. 4,453,834. According to the third technique, sensitivity is digitally corrected with a variable frequency by a preset counter. U.S. Pat. No. 4,453,834 describes an arrangement using an IC sensor as a temperature sensor with linear characteristics. The B constant correction (i.e., sensitivity correction) results are represented by polygonal lines b3 and c3 of FIG. 3. By correcting the angles .theta.1 and .theta.2 of the polygonal lines b2 and c2 with respect to the polygonal line a1 having the reference temperature sensitivity characteristics, the characteristics represented by the polygonal lines a1, b3 and c3 are rendered identical.
As described above, in order to manufacture thermistor thermometers with high productivity at low cost, different characteristics of the thermistors are matched by variation correction with the reference temperature sensitivity characteristics, and a counter having the linear characteristics is used.
The following conventional techniques can be proposed to perform variation and linear correction of the thermistor in the conventional thermistor thermometer:
(1) A series- and parallel-connected resistor array 2 is connected to the thermistor 1, as shown in FIG. 4. The resistance of the resistors R1 to R4 constituting the resistor array 2 are selectively adjusted in association with the temperature sensitive resistance of the thermistor 1. The temperature sensitivity curve of the thermistor 1 is changed to simultaneously perform linear and sensitivity correction. Reference resistance correction is performed by adjustment of the oscillating capacitor C0 or digital processing.
(2) As shown in FIG. 5, a resistance of the thermistor 1 is converted by an A/D converting means 3 to digital data. The digital data is fetched by a microprocessor 4. Corrected digital data is also fetched by the microcomputer 4 through a data setting means 5. The digital data corresponding to the resistance of the thermistor 1 from the A/D converting means 3 is subjected to one or both of linear and sensitivity correction operations, thereby obtaining the temperature data. The corrected digital data is prepared as data for standardizing the temperature sensitivity characteristics of the specific thermistor 1 in association with the temperature sensitivity characteristics.
Reference resistance correction is the same as technique (1).
(3) As described in U.S. Pat. No. 4,453,834, sensitivity correction is digitally performed by a variable frequency divider (in the case of U.S. Pat. No. 4,453,834, linear correction need not be performed since the temperature sensor has linear characteristics).
In technique (1), it is very difficult to adjust the resistances of the resistors R1 to R4 for providing predetermined temperature sensitivity characteristics. In addition, adjustment is cumbersome and time-consuming, resulting in low productivity. In spite of such difficulties, ideal characteristics cannot be obtained due to an irregular characteristic curve. Sufficient correction precision for a thermometer cannot be obtained. In addition, when thermometers are automatically manufactured, a compact lightweight thermometer cannot be achieved due to a complex circuit arrangement. As a result, a handy thermistor thermometer at low cost cannot be easily manufactured.
In technique (2), correction precision is satisfactory. However, the microcomputer and the A/D converting means are required, so that the thermometer device as a whole becomes large and expensive. Therefore, it is difficult to realize a handy, inexpensive thermometer.
In technique (3), correction precision is better than that in technique (1), and the arrangement is simpler and the price is lower than those in technique (2). Technique (3) is most suitable for achieving an inexpensive compact thermistor thermometer. However, "variation" correction of the temperature sensitivity characteristics of the thermistor in technique (3) presents the following problem.
In a conventional thermometer based on technique (3), reference resistance correction and sensitivity correction are independently performed.
The method of correcting variations in sensitivity in technique (3) will be described with reference to FIG. 6. When a thermistor having a temperature sensitivity characteristic curve d in FIG. 6 is corrected to provide the reference temperature sensitivity characteristic curve a, reference resistance correction is performed at the reference temperature T0 as the central temperature (37.degree. C. in the clinical thermometer) of the possible operating temperature range. As indicated by a curve d1, the reference temperature is matched with the number N0 of the pulses. Sequentially, by performing digital sensitivity correction by using a variable frequency, the curve d1 is shifted to a curve d2 having the same slope as that of the curve a. In this case, the curve d2 is found to be given such that the pulse number corresponding to the reference resistance is shifted from N0 to Nd. In this manner, when digital sensitivity correction is performed throughout the temperature sensitivity characteristic curve of the thermistor, reference resistance correction influences sensitivity correction. Therefore, two types of correction must be alternately repeated, and adjustment is complex.